1. Field of the Invention
This invention relates to benzoxazole derivatives which are key intermediates useful in producing 2-amino-5-nitrophenol derivatives in which various nucleophilic groups are introduced at the 4 position.
2. Description of the Prior Art
2-Amino-5-nitrophenol derivatives are general-purpose compounds for use as industrial starting materials, and can be converted into synthetic intermediates of higher added value by reduction of the nitro group. The compounds have the o-aminophenol structure which may serve as a reducing agent, and the degree of reducibility can be arbitrarily controlled by introduction of a suitable substituent and conversion of the nitro group into other functional group. In this sense, the derivatives are important as diversity of reducing agents or antioxidants or synthetic intermediates which are convertible into physiologically active compounds by modification of the nitrogen atom.
2-Amino-5-nitrophenol derivatives are also important as synthetic intermediates for cyan-image-forming couplers in the field of photographic chemistry. In recent years, it has been found that 2,5-diacylaminophenol cyan-couplers show good color restoration upon development and the resulting dyes have good fastness to heat and/or humidity (see, for example, Japanese patent application (OPI) Nos. 110530/78, 163537/80 corresponding to U.S. Pat. No. 4,299,914, 29235/81 corresponding to U.S. Pat. No. 4,304,844, 55945/81, 31953/84 corresponding to U.S. Pat. No. 4,463,086 and 31954/84 also corresponding to U.S. Pat. No. 4,463,086, and U.S. Pat. Nos. 4,124,396 and 4,341,864). It has been also found that dyes produced from 2-phenylureideo-5-acylaminophenol cyan-couplers are excellent in color restoration upon development, absorption wavelength, and fastness to heat and/or humidity (see, for example, U.S. Pat. Nos. 4,333,999 and 4,427,767 and Japanese patent application (OPI) Nos. 204543/82, corresponding to U.S. Pat. No. 4,451,559, 204544/82 also corresponding to U.S. Pat. No. 4,451,559 and 204545/82). Accordingly, 2-amino-5-nitrophenol derivatives have attracted attention for use as synthetic intermediates for those couplers.
Photographic couplers may be broadly classified with respect to the hue of dye developed dye. They may also be stoichiometrically classified into two broad classes, i.e. 2-equivalent couplers and 4-equivalent couplers. While 4-equivalent couplers require four moles of silver halide to be developed into one mole of dye, 2-equivalent couplers have a split-off group at the coupling position thereof and can form one mole of dye using two moles of silver halide. For this reason, it is known that 2-equivalent couplers are more beneficial from the standpoint of silver savings. With regard to cyan couplers, for instance, 2-equivalent couplers have such a high color-developing speed that photographic sensitivity is much improved (see, for example, U.S. Pat. Nos. 3,476,563, 3,617,291, 3,880,661, 4,052,212 and 4,147,766, British Pat. Nos. 1,531,927 and 2,006,755, and Japanese patent application (OPI) No. 32071/80 corresponding to U.S. Pat. No. 4,254,212, 1938/81 corresponding to U.S. Pat. No. 4,296,199 and 27147/81 corresponding to U.S. Pat. No. 4,296,200).
As recent color negative films increase in sensitivity, 2-equivalent couplers of high color developing speed, in which slit-off groups are introduced at the coupling position, have been employed in large amounts. Accordingly, of increasing importance are 2-amino-5-nitrophenol derivatives and processes for preparing such derivatives.
As explained above, 2-amino-5-nitrophenol derivatives in which substituents are introduced into the benzene ring are important as industrial starting materials, reducing agents, and intermediates for preparing cyan couplers in photographic chemistry. Preparation of these derivatives is described, for example, in U.S. Pat. No. 3,880,661, and Japanese patent application Nos. 145333/83, 157423/83 corresponding to U.S. Pat. No. 4,579,813, 158470/83, 157424/83, and 199,696/83. One such example may be represented by the following reaction formula (a) ##STR1## wherein Z represents a nucleophilic group.
The above substitution reaction per se is known as an aromatic nucleophilic substitution reaction. This reaction is described in detail, for example, in Jerry March "Advanced Organic Chemistry" (second edition, (1977) Mcgraw-Hill Kogakusha, Ltd.), chapter 13 entitled "Aromatic Nucleophilic Substitution" pp. 584-595. In the Journal of American Chemical Society, Vol. 79, p. 385 (1957), J. F. Bunnet et al report that the reaction between 2,4-dinitrobenzene derivatives and pyridine proceeds about 3300 times more rapidly when the split-off group is a fluorine atom than when the split-off group is a chlorine or bromine atom.
However, the conventional processes of synthesis, as exemplified as formula (a), have a number of disadvantages.
(1) The fluoro derivative 1 used as the starting material, is obtained by five steps starting from p-fluorophenol and is thus complicated in preparation steps.
(2) The starting p-fluorophenol is not readily available and is expensive.
(3) Because of fluorine ions generated by the reaction, additional plant investment is necessary for safety and water disposal.
(4) The type of material for reactors is limited.
These disadvantages place a serious limitation on mass production. On the other hand, in order to overcome the above disadvantages, it is a matter of course that the reaction formula (a) is effected using, instead of fluoro derivatives (Compound 1), chloro derivatives (Compound 3 below) as described in U.S. Pat. No. 3,880,661. ##STR2##
This chloro derivative is prepared from 4-chloroaminophenol which are inexpensively available in large amounts. As expected, however, the chloro derivative is much less reactive than the fluoro derivative, thereby giving low yields. Even though the reaction corresponding to formula (a) is carried out using Compound 3 in the presence of a catalyst such as, Cu, CuI, CuI.sub.2, CuCl.sub.2, CuBr.sub.2 and CuO (Ullmann reaction, Fanta, Synthesis, 9, 12, 1974), the yield of the desired product (Compound 2) is about 7% maximum with the balance being by-products with unidentified structures.